As is known in the art, an optical amplifier is a device that increases amplitude of an input optical signal that is launched into the amplifier fiber together with pump light. If the optical signal at the input to the amplifier is monochromatic, the output will also be monochromatic with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode, glass fiber having a core doped with an active material or dopant which is excited by pumping the fiber with optical energy at predetermined wavelengths within an absorption band of the fiber dopant. The input signal is provided to the gain medium, via an optical coupler with low insertion loss. The pump energy is combined with the input signal within the coupler and launched into the fiber core. If the fiber is a double clad fiber, then the pump light is launched into the inner cladding of the fiber while the signal is launched into the core. As is well known in the art, the pump energy produces a population inversion in the dopant, and the input signal is amplified by stimulated emission to produce an amplified output signal which is emitted from the other opposite end of the fiber. Such fiber amplifiers can be unidirectionally pumped with pump sources at one end or bi-directionally pumped with pump sources provided at opposite ends of the fiber, one co-propagating with the signal and the other counter-propagating with the signal. The employment of bi-directional pumps provides for higher power output and more power conversion efficiency in the fiber. A representative example of a bi-directionally pumped fiber amplifier is illustrated in the patent to Huber, U.S. Pat. No. 5,140,456. Typically, as taught in patent '456, co-pumping with 980 nm light provides for lowest noise figure performance at the amplifier input side of the erbium doped fiber amplifier (EDFA), and counter-pumping with 1480 nm light provides for highest conversion efficiency from the amplifier output side. In addition, crosstalk between 980 nm and 1480 nm pumps is minimal because their respective frequencies are so widely separated. Patent '456 also discusses the length of the amplifier fiber in context of fiber doping concentration level an a level of inversion necessary for amplifier operation determined by fiber emission and absorption. However, absent from the patent is any treatment for achieving maximum uniform conversion along the length of the fiber or consideration of the absorption profile along the length of the fiber relative to its chosen length.
Power conversion efficiency has been studied in rear earth doped fiber amplifiers since patent '456 indicating that the difference between forward and backward pumping schemes is primarily effected by the saturating effect by amplified spontaneous emission (ASE), and that highest conversion efficiency in forward and backward pumped, highly saturated Er doped fiber amplifiers is most closely reached with backward pumped Er doped fiber amplifiers. See the article of E. Desurvire, "Analysis of Gain Difference Between Forward- and Backward-Pumped Erbium-Doped Fiber Amplifiers in the Saturation Regime", IEEE Photonics Technology Letters, Vol. 4(7), pp. 711-714, July, 1992.
Optical fiber amplifiers are typically employed in a wide variety of applications, including, but not limited to, amplification of weak optical pulse such as those that have traveled through many kilometers of fiber length in optical telecommunication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are typically formed. For example, the EDFA in patent '456 operates in a three level mode and is capable, when pumped with a wavelength of 980 nm, of amplifying a 1550 nm signal. Since 1550 nm is the lowest loss wavelength of conventional single mode silica glass fibers, EDFAs are well suited for fiber systems that propagate optical signals having wavelengths around 1550 nm.
One significant concern with fiber amplifiers is their power conversion efficiency, i.e., the optical amplification achieved with a given pump power level. In order to increase the conversion efficiency of an amplifier, the amplifier is typically pumped with pump energy that propagates in a direction counter to the input signal propagation direction, as previously indicated.
Also, of concern is the noise figure of the amplifier, which generally defined as the ratio of the output noise power to the amplifier input noise power. In order to decrease the noise figure of an amplifier, the amplifier is typically pumped with pump energy that propagates in the same direction as the input signal propagation direction, as previously indicated.
What is further desired is to provide (a) power scaling of the pump power utilizing pump sources that have different wavelengths for purposes of WDM combining while selecting their wavelengths of operation within the absorption band of an amplifier relative to its peak absorption wavelength to produce a particular composite absorption profile relative to the selected fiber length so as to achieve uniform conversion of the pump energy along the fiber length, and (b) noise figure optimization by judicious choice of co-pumping and counter-pumping with different wavelengths to take advantage of the wavelength-dependent pump absorption of the fiber amplifier
Thus, it is an object of this invention to provide a fiber amplifier further optimized for power conversion and/or noise figure.
It is another object of this invention to utilize co-pumps and counter-pumps in a manner to tailor the pump gain distribution or the absorption profile along the length of the fiber to achieve different absorption/gain characteristics thereacross.